Non-metallic fibers useful for rubber reinforcement and especially for tire reinforcement include relatively high denier nylons, rayon, as well as polyester. A particularly preferred polyester is poly(ethylene terephthalate). Because mechanical properties are important, it is typical to employ yarns made up of highly oriented filament which may be prepared in a variety of ways. With respect to poly(ethylene terephthalate) one process involves spinning the yarn to a relatively low birefringence (&lt;0.009) and then drawing the yarn. For example see: U.S. Pat. Nos. 3,216,187 or 3,361,859. Another process involves spinning the yarn to a relatively higher birefringence (i.e. 0.009) and drawing off-line. For example see: U.S. Pat. No. 4,973,657. Another process involves spinning the yarn and subsequently draw-twisting the yarn. The preferred process involves spinning the yarn to a relatively high birefringence (i.e. 0.009) and drawing in-line. For example see: U.S. Pat. Nos. 4,101,525; 4,195,052, 4,414,169; 4,690,866; 4,551,172; 4,827,999; 4,491,657, 5,067,538, 5,132,067; and 5,234,764. Preparation of the yarn is merely the first step, since the yarns must be suitably adhered to the rubber components in order to impart the desired properties to the end product.
In connection with tire manufacture, it is typical to manufacture specialized fabrics which are coated with rubber for use in plies, breakers, chippers and belts. Initial manufacture consists of spinning and drawing the yarns as noted above as well as applying a finish. The yarn is twisted into plies, cabled into cords, woven into fabrics, and treated with an adhesive dip prior to being coated with rubber. To facilitate processing with adhesives and calendaring with rubber, the cables are woven into a fabric, for example of 23-35 ends per inch with a minimum number of filament yarns or staple fiber pick threads, also called fill threads or weft. The fabric is dip-coated with an adhesive which bonds with rubber. The adhesives are most commonly aqueous systems including rubber latex, resorcinol and formaldehyde which are allowed to partially react before dip application.
The multi-step yarn pre-treatment process involved in tire manufacture is of course expensive, both in terms of capital expenditure and processing costs; especially in connection with weaving, adhesive application, and environmental control costs, which expenses are interrelated inasmuch as the weaving step is required in large part to facilitate adhesive application.
Bilayer spinning of synthetic fibers has been employed to provide fibers with a surface layer more suitable for a given end use. Rayon/nylon bicomponent fibers are shown, for example, in U.S. Pat. No. 5,272,005; while U.S. Pat. No. 5,227,109 discloses bicomponent fibers with a poly(ethylene terephthalate) core and a copolyester sheath. Perhaps more notably, U.S. Pat. No. 4,987,030 shows a polyester core/nylon sheath bicomponent fiber useful as rubber reinforcement. Additional multilayer fibers and cords may be seen in the following U.S. Pat. Nos.: 4,520,066; 4,129,692; 4,024,895; 3,839,140; 3,645,819.